In a typical optical storage system, information is generally stored in spiral or concentric circular tracks on a rotating disk. The track density of an optical storage system is typically an order of magnitude greater than that of a magnetic storage system. In the case of a read-write optical system, a polarized laser beam may be used for both storage (writing) and retrieval (reading) of the data. The system includes an optical head carrying an objective lens that converges the laser beam on a small spot on a selected track on the disk surface and an actuator that moves the head radially over the disk surface for access to different tracks.
Because of the increased track density, optical storage systems typically utilize a two-stage track access system: (i) a coarse track-access mechanism for large scale movement of the laser beam, i.e., over a relatively large number of tracks, and (ii) a fine-positioning mechanism to move the beam over a limited number of tracks and to follow the center line of the selected track. The laser beam is collimated and, ideally, it is directed through the front focal point of the objective lens so that when the lens converges the beam at the disk surface, the light reflected from the surface returns along the path of the incident laser beam. This helps to provide the accuracy required in the servo system that maintains the converged beam on the centerline of the track on which the head is positioned. This function is also termed "track-following."
Coarse track access is accomplished by radial displacement of the entire optical head. Fine track access is typically accomplished by means of a 2-dimensional voice coil actuator mounted on the optical head and moving the objective lens lateral across tracks on the disk surface. The actuator also moves the lens axially for focusing of the beam. This results in a massive head assembly which hinders performance of coarse movement and which requires displacement of the entire head. Moreover, the space occupied by the fine-track actuator prevents close spacing of the disks if a multiple-disk system is contemplated.
The mass of parts on the head assembly may be decreased by using a stationary fine-track actuator. This "remote" approach, which has been suggested for track following, may involve the use of a remote lens that is moved transversely with respect to the optical axis to change the position of the collimated laser beam at the objective lens; alternatively, a mirror mounted to a high-speed galvanometer may be rotated to change the angle of incidence of the beam at the objective lens. This latter arrangement has not, however, been successfully implemented for fine-positioning operations, which require a substantially greater angular displacement of the beam than does track-following.
More specifically, because the track-following actuator is relatively far removed from the objective lens, even small angular adjustments of the laser beam result in a large transverse displacement of the beam at the objective lens. This typically leads to beam "clipping" and an accompanying loss of beam energy. In addition, the displaced beam is far removed from the front focal point of the objective lens. As a result, the reflected beam does not return along the path of the incident beam, a phenomenon known as "beam walk-off", and the position sensor used in the track-following servo system has a large offset error. For similar reasons, fine-positioning by means of a remote lens is impractical. In order to minimize these problems, previously known systems have generally avoided use of remote fine positioning mechanisms.
Several possible remote fine positioning mechanisms might be employed to resolve these problems. One approach involves the use of a relatively large (and expensive) optical prism having parallel faces in conjunction with the rotating mirror. The prism is rotated to translate the laser beam and thereby center it on the front focal point of the objective lens regardless of the angular displacement of the beam by the mirror. This requires an additional actuator to rotate the prism. In addition, the large prism responds slowly to control signals, thereby reducing the bandwidth of the system.
An alternative approach involves the use of two galvanometer-rotated mirrors positioned in tandem in the path of the laser beam. With proper positioning of the mirrors and control of their movements, the direction of the beam can be changed without a corresponding translation of the beam away from the front focal point of the objective lens. However, because two active, moving elements are used, a complex servo system is required to control this mechanism. Also, the back-to-back galvanometer configuration is inefficient because it requires relatively large angle excursions to produce small, compensated angle excursions at the objective lens.
Therefore, it is desirable to provide an efficient remote fine positioning mechanism for a high-performance, optical storage system.
It is also desirable to provide a remote fine positioning mechanism capable of track selection as well as track-following operations.
In addition, it is desirable that the remote fine positioning mechanism exhibit minimal beam clipping and walk-off, yet maintain low-cost, high-performance characteristics.